Typical water runoff contains a surprisingly large amount of oil and other contaminants. The resulting contamination of natural receiving water incurs enormous annual costs, both financial and environmental. For example, a government study in one published article showed that storm water sampled from street sites contained an “event mean concentration” of 2.2 mg. of oil per liter of runoff water. Shepp, “Petroleum Hydrocarbon Concentrations Observed in Runoff from Discrete, Urbanized Automotive-Intensive Land Uses,” Watershed '96. If one meter of rain per year falls on a street 10 meters wide, then at that observed mean rate, the annual runoff from each kilometer of street will contain about 275 liters of hydrocarbons.
To mitigate this problem, various types of filters have been developed for use in water drains. For example, U.S. Pat. No. 5,849,198 to Sharpless discloses a filter cartridge containing oil absorbing material, which is suspended below the grate of a storm drain by flexible suspension elements such as chains. The top of each of the flexible suspension elements terminates at an attachment element that engages the grate of the storm drain.
To maintain its effectiveness, a runoff filter or catchment basin requires regular maintenance. Filter modules or cartridges containing filter media must be replaced or serviced when the media becomes saturated. The Sharpless patent teaches a replacement technique in which the filter cartridge is lifted up and out of the drain's catch basin as the grate is removed. This process requires simultaneous lifting of the weight of the grate and the filter cartridge. Typically, a filter cartridge being replaced contains accumulated trash or saturated filter media or both, and it can be very heavy. Considerable force is needed to dislodge a heavy metal grate from its catch basin and lift the grate and the filter cartridge out of the basin together.
In view of these and other problems associated with conventional runoff filters, the need remains for improved runoff filters that can be more easily maintained while still accommodating excess runoff under storm conditions. A further need remains for runoff filters that can be more efficiently manufactured, stored, and shipped and disposed of with reduced environmental consequences.